Fix backend url

* BackendService generates urls such as 'http://example.com//api/blocks', but these url doesnt work with AWS ALB

frontend/src/app/backend.service.ts

@@ -108,7 +108,7 @@ export class BackendService {
   private request(url: string, params?: HttpParams): Observable<any> {
     return this.getConfig().pipe(
       mergeMap((config: Config) => {
-        return this.http.get(`${config.backendUrl}/${url}`, { params });
+        return this.http.get(`${config.backendUrl}${url}`, { params });
       })
     );
   }